This invention relates to crystal oscillator circuits and operating methods thereof and, more particularly, to capacitive load circuits for crystal oscillators and operating methods thereof.
Crystal oscillators are widely used in electronic devices. For example, a satellite television set-top box may include a voltage-controlled crystal oscillator (VCXO) circuit that is used to tune a reference signal of a receiver circuit of the set-top box. The set-top box's receiver circuit may include a control loop that generates a pulse-width-modulated (PWM) frequency command signal that is filtered by a low-pass RC filter and applied to an analog VCXO that controls the frequency of a signal generated from a crystal oscillator responsive to the filtered signal. In particular, the VCXO control circuit may capacitively load the crystal oscillator to “pull” its output frequency over a range around its resonant frequency. An example of a conventional VCXO circuit is described in U.S. Pat. No. 5,764,111 to Bal et al.